This invention pertains to the art of extrusion dies and more particularly to extrusion dies for synthetic resin or polymeric materials.
The invention is particularly applicable to a so-called underwater extrusion die for use in pelletizing synthetic resins or polymeric materials and will be described with particular reference thereto. However, it will be appreciated by those skilled in the art that the invention has broader applications and may be adapted to use in other environments.
In extruding synthetic resins and similar materials to form pellets, the extruded material is cut into pellet form by a knife which periodically wipes across the external face of an extrusion die. The interior of the die is maintained at elevated temperatures for retaining the resins in a generally fluid or flowable state. However, the external face of the die where the wiping or cutting action takes place must be maintained at much lower temperatures so that the material being extruded will solidify substantially simultaneous with its exit from the die. Typically, the external die face is submerged in a cooling fluid bath such as water or the like to achieve necessary cooling. The resultant uneven heating and cooling causes a great deal of expansion and contraction of the die which can adversely affect die operation and effective die life.
In this type of extruding equipment, it is considered particularly advantageous to design and construct the die in a manner such that at least the external die face which periodically receives the knife wiping or cutting action has a much greater hardness than the die body itself for reducing die wear. Hard facing tungsten carbide tiles or the like are typically used for facing the external die face with one such carbide facing arrangement being shown and described in the commonly assigned U.S. Pat. No. 3,271,822. While this particular patent has helped solve problems encountered with wear generated by the knives running against the die face, certain operational difficulties are still encountered during extruding operations.
More particularly, such dies have not operated in an entirely efficient manner due to the rapid heat conductivity of the face which conducts heat outwardly away from the die body. Because of this, the synthetic resin or polymeric material passing through the extrusion holes or ports can often cool sufficiently to solidify and thereby plug the ports. This problem is referred to in the trade as "freeze off" and is caused simply by the inability to maintain a sufficient temperature in the area surrounding the extrusion ports adjacent the facing. The occurrence of freeze off results in an undesired reduction in production capacity. Ultimately, the die must be removed from production for purposes of cleaning the plugged ports.
There have been some previous efforts at overcoming freeze off which have met with varying degrees of success. For example, insulating type coatings on the facing have been used but this approach substantially shortens effective face life. In addition, combinations of hard facing and insulating coatings on the face have helped reduce the problem somewhat, but this approach similarly has the drawback of limited operational life. Still further, some users have utilized a so-called heat exchanger die plate but here too, freeze off remains as a recurring difficulty.
Since tungsten carbide facings provide the best production life for these dies, it has been considered extremely desirable to develop means for more effectively insulating the die body from such facing constructions. Preferably, the manner of insulation should not cause any loss of effective die life and substantially eliminates loss of production capacity due to freeze off. The subject invention is deemed to meet the aforementioned specific needs as well as others commonly associated with this general type of extrusion die.